Attempts are being made to manufacture smaller, lighter and more energy-saving refrigerators. Today, an increasing number of refrigerators are used throughout the year. Another interesting fact is that reciprocating compressors that have been used in air conditioners for automobiles and houses are being replaced by rotary compressors. All of these phenomena are factors that put frictional and lubricated areas in the compressor (e.g., bearing, piston, seal unit and vane) under severer conditions, and it is important to take care to prevent excessive wear and seizure. More importantly, the friction loss in the lubricated area must be reduced. For these reasons, one requirement for lubricants to be used in refrigerators in Flon atmosphere is that they have good lubricating properties (e.g., wear resistance and seizure resistance) and low viscosity.
One must fully recognize the fact that lubricants for use at lubricated areas in apparatus that are exposed to a sealed Flon atmosphere and other lubricants (e.g., gasoline engine oil, diesel engine oil, industrial lubricants such as cutting oil, hydraulic oil, gear oil and other oils that are used in an oxygen-containing atmosphere such as air) require entirely different design philosophies in improving their lubricating properties such as wear resistance and seizure resistance. The former type must exhibit lubricating properties in Flon atmosphere whereas the latter must have lubricating properties in an oxygen-containing atmosphere. The extreme pressure tests specified in JIS or ASTM (e.g., the four ball test in ASTM D2266-78 or Falex test in ASTM D3233-73) assumes testing in an air atmosphere whether the lubricant under testing is actually used in such atmosphere or not. So, one who looks at the test data is apt to overlook the effect of the testing atmosphere in his evaluation of the performance of the lubricant. Some improvement in the wear resistance and load resistance is achieved by adding certain sulfur compounds (e.g., sulfurized olefin and sulfurized sperm oil) to the lubricant, but natural organic sulfur compounds in mineral oils are not effective for the wear resistance and load resistance.
In our study on the lubricant for use in machines operated in a sealed Flon atmosphere, we have found that the lubricating atmosphere itself has great effect on the wear behavior and that the presence of natural sulfur compound in mineral oils help provide improved wear resistance in a Flon gas atmosphere. Table 1 compares two mineral oils that were desulfurized to different extents and indicates that the presence of sulfur produces a large wear scar in air but that it proves effective against wear in a Flon atmosphere.
TABLE 1 ______________________________________ Abrasion Scar Atmosphere (mm) ______________________________________ Mineral oil (sulfur = 0.03 wt %) Air 0.51 Mineral oil (sulfur = 0.03 wt %) Argon 0.42 Mineral oil (sulfur = 0.03 wt %) Flon (R-22) 0.43 Mineral oil (sulfur = 0.27 wt %) Air 0.62 Mineral oil (sulfur = 0.27 wt %) Argon 0.38 Mineral oil (sulfur = 0.27 wt %) Flon (R-22) 0.28 ______________________________________
As a result, we have found that the need of lower viscosity that had been considered incompatible with good lubricating properties can be met and a lubricant that retains good lubricating properties down to an extremely low viscosity range can be obtained by modification of sulfur content. This finding is not obvious at all in the prior art and is particularly unique to a Flon atmosphere.
So far, naphthenic mineral oils have been used with advantage as refrigerating oils, but because naphthene base crude oils are not easily available these days, mineral oils derived from more easily available paraffin or mixed base crude oils are preferred. These mineral oils do not have high ability to dissolve Flon at low temperatures, and to solve this problem, Japanese Patent Application (OPI) No. 139608/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") teaches the use of alkylbenzene. It also teaches the use of a phosphite ester for providing improved wear resistance. Japanese Patent Application (OPI) Nos. 54707/77 and 127904/77 teach that alkylbenzene lowers the critical dissolution temperature and provides higher heat resistance. But the lubricating properties, such as wear resistance, of alkylbenzene are not as good as those of mineral oils, and they fail to achieve the desired lubrication in compressor parts. The phosphite ester undergoes hydrolysis in the presence of a trace amount of water.